This invention relates to an in-plane switching mode liquid crystal display (IPS-LCD), and more particularly to an IPS-LCD having an electrode for preventing a static electricity generated from an upper substrate.
Recently, liquid crystal displays (LCDs) with light, thin, low consumption characteristics are used in OA equipments and video units and the like. There typically are a twist nematic (TN) mode and a super twist nematic (STN) mode as a LCD driving method. Although TN-LCDs and STN-LCDs have put to practical use, they have a drawback of a very narrow viewing angle. In order to solve the problem, IPS-LCDs are proposed. IPS-LCD includes a lower substrate where a pixel electrode and a counter electrode are disposed, an upper substrate having no electrode and a liquid crystals are sandwiched between the substrates.
IPS-LCD in the prior art is shown in FIGS. 1 and 2. A lower substrate 10 and an upper substrate 11, which are made of glass material, are opposed being spaced in a predetermined distance. A LC layer 15 is sandwiched between the lower and upper substrates 10 and 11 and LC molecules 17 included in the LC layer 15 whose anisotropy of dielectric constant is positive. On an inner surface of the lower substrate 10, a gate bus line which a scanning signal is applied thereto and a data bus line which a display signal is applied thereto are crossed over each other to define an unit pixel area p. At the intersection of the gate and data bus lines, a thin film transistor (TFT) 105 for switching device is disposed.
A counter electrode 3 for supplying with a common signal is disposed at the unit pixel area P of the inner surface of the low substrate 10. The counter electrode 3 has a comb shape in plane view and includes a plurality of branches 3a, 3b and 3c which are spaced in a selected distance at each other and are parallel to the data bus line 103 and a bar 3x which is connected to one-sided edges of the branches 3a, 3b and 3c and is parallel to the gate bus line 101. The pixel electrode 7 which is connected to the TFT and a display signal is applied thereto, is disposed at the unit pixel area P of the inner surface of the low substrate 10. The pixel electrode 7 has also a comb shape and the pixel electrode 7 and the counter electrode 3 are arranged with a teeth form. The pixel electrode 7 includes a plurality of branches 7a and 7b which are parallel to the data bus line 103, each branch being disposed between the branches of the counter electrode 3 and a connection part 7x which is connected to the TFT and one-sided edges of the branches 7a and 7b and is parallel in part to the gate bus line 101.
A gate insulator 5 is formed between the counter electrode 3 and the pixel electrode 7 so as to isolate them. Over the resultant surface of the lower substrate 10, a first alignment film 9 which determines the initial alignment direction of the LC molecules 17, is formed. A color filter (not shown in drawings) is disposed on an inner surface of the upper substrate 11 and a second alignment film 13 is disposed on the inner surface of the upper substrate 11 including the color filter. Herein, the first and the second alignment films 9 and 13 are homogeneous alignment film having a pretilt angle of below 10.degree.. The first alignment film 9 is rubbed to the direction which makes the angle of about .+-.45.degree. to the gate bus line and the second alignment layer 13 is rubbed to the anti-parallel direction which makes the angle of 180.degree. against the rubbing direction of the first alignment film 9. At this time, the rubbing direction of the first alignment film 9 is varied with anisotrophy of dielectric constant of the LC molecules 17.
A polarizer 19a which firstly polarizes an incident light, is disposed on an outer surface of the lower substrate 10 and an analyzer 19b which secondarily polarizes the light passing through the LC layer 15, is disposed on an outer surface of the upper substrate 11. The polarizer 19a is attached to the lower substrate 10 so as to coincide its polarization axis with the rubbing direction of the first alignment film 9 and the analyzer 19b is attached to the upper substrate 11 to coincide its polarization axis with the polarization axis of the polarizer 19a.
The IPS-LCD as above-constructed operates as follows. When an electric field is not generated between the counter electrode 3 and the pixel electrode 7, the LC molecules 17 are arranged so as to run the long axes thereof parallel to the rubbing axes of the first and second alignment films 9 and 13 and so as to coincide the long axes of the LC molecules 17 with the polarization axis of the polarizer 19a, thereby turning to the dark state. When the voltage difference is generated between the counter electrode 3 and the pixel electrode 7, because both of the counter electrode 3 and the pixel electrode 7 are formed on the lower substrate 10, the in-plane field E which is parallel to the surface of the lower substrate 10, is generated between the counter electrode 3 and the pixel electrode 7. Accordingly, the LC molecules 17 are twisted so as to coincide the long axes thereof with the electric filed direction, thereby occurring the angle difference between the polarization axis of the polarizer 19a and the long axes of the LC molecules 17. Therefore, the light passing through the polarizer 19a passes the LC layer 15 and the analyzer 19b, thereby turning to the white state.
The prior IPS-LCD has advantages as follows. The alignment films are rubbed using a rubbing cloth to determine the rubbing direction thereof. In rubbing, the rubbing friction is caused between the surface of the alignment layer and the cloth to generate the static electricity on the surface of the alignment film. Besides, a protection film is attached on the surface of the analyzer so as to protect a screen in LCDs of the end-products. The protection film is detached from the LCDs in user's utilization of LCDs. At this time, charges are instantaneously electrified between the analyzer and the protection film. The static electricity is typically discharged by the electrodes of LCDs. However, the electrodes are not formed on the upper substrate in IPS-LCDs and the static electricity generated in the upper substrate is not discharged with ease and remains on the surface of the upper substrate. Therefore, the parasitic electric field is generated between the remaining static electricity in the upper substrate and electrodes of the lower substrate, thereby degrading the display characteristics of the LCDs.